Traditionally, multi-functional mobile communication terminals and other electronic devices commonly known as mobile phones and smartphones use piezoelectric elements called “piezoelectric speakers” as sounding elements for audio output. With traditional electronic devices, a piezoelectric element is placed inside a housing, and the housing has sound-passing holes to transmit sound to the outside by means of air conduction. On the other hand, technology has been proposed in recent years, whereby sound is transmitted by installing a piezoelectric element on the rear face side of a panel placed on the principle face of the housing, and then causing the panel to vibrate. Behind the proposal of such technology are various needs, such as a need to eliminate sound-passing holes so as to enhance waterproof property, a need to transmit sound not only by means of air conduction using air as the medium, but also by means of bone conduction, and a need to implement haptic technology that transmits vibration of a panel to the user tactually. Here, a “panel” means a touch panel that only has touch function and no display function, a touch panel that has both display function and touch function, or a protective panel designed to protect a liquid crystal display or other separate display.
One traditional electronic device adopting such technology is described in Patent Literature 1. The electronic device described in Patent Literature 1 has a housing, a panel of roughly rectangular shape whose edges are supported in the housing, and a piezoelectric element of roughly rectangular solid shape installed on the rear face side of the panel. The piezoelectric element is installed in such a way that its longitudinal direction intersects at right angles with the longitudinal direction of the panel, and in particular, it is positioned in such a way that the vibration amplitude of the panel becomes the highest at the installed position of the piezoelectric element. To be specific, the piezoelectric element is installed at a position near one end of the panel in the longitudinal direction.